In general, non-solvent type varnish does not contain a solvent whereby all of the components can be effectively used without loss and no volatile matter is vaporized to be desirable from the viewpoint of hygienic safety.
Accordingly, the non-solvent type varnishes have been mainly used instead of the conventional solvent type varnishes.
Unsaturated polyester resins and epoxy-modified resins have been used in the non-solvent type varnishes. These resins are easily deteriorated at high temperature whereby the resins are not durable for a long time, disadvantageously.
For example, epoxy compounds have been used by hardening with a hardener such as amino compounds and acid anhydrides as resins having suitable electrical characteristics, dimensional stability and chemical resistance in various fields however heat resistance of the epoxy resin is not satisfactory.
On the other hand, it has been known that maleimide compounds are polymerized to produce resins having high heat resistance. For example, it has been known to produce three dimensional polyimides by polymerizing only N,N'-di-substituted maleimide under heating in French Pat. No. 1,455,514.
However, the three dimensional polyimides have high cross linkages whereby cracks easily form upon heating and cooling. Accordingly, these polyimides are not suitable as resins for casting, disadvantageously.
Moreover, it has been proposed to obtain resins having high heat resistance by combining the epoxy resin with maleimide compound in Japanese Patent Publication No. 12600/1974. The resin obtained by the combination of the epoxy resin and maleimide compound have high heat resistance, however, the varnishes having the resin having disadvantages to precipitate the resin by maintaining for a long time because of low solubility of the maleimide compound. Moreover, the heat shrinkage of the resin in the hardening step is high whereby the adhesion on surface of a coil is not suitable in an immersing step or a casting step for a large coil and the resins are peeled off or cracked to cause a trouble in a practical used.